a defense of knowledge. it's not clear at all at this point whether dzhokhar knew the ramifications of what he was doi doing, or knew what if anything was in the backpack that he was carrying. we don't know what his brother told him. we don't know what he agreed to. we don't know if he felt under pressure or threat from his brother or from others. all of these are open questions that will need to be explored. >> and, tamar, this is one of the issues i wondered right away. when the statements were made that there's evidence of video of dzhokhar dropping a backpack and walking away callously, that is a very, very specific piece of evidence if in fact it's true. would that be the argument that a defense attorney perhaps someone like you would think of right away, does he even know what's in that backpack given the kind of person we now know his alleged co-conspirator to have been? >> absolutely. you know, knowledge, as i said, will be a key element here. and it will be something that the u.s. government is goin